The science of X-ray astronomy was born on 18 June 1962. That was the day a team led by the Italian-American astronomer Riccardo Giacconi fired a crude X-ray telescope to the edge of space on board a sounding rocket.

In the 15 minutes or so the rocket hung above the obscuring effect of the atmosphere before plunging back to Earth, the telescope peered for the first time at the X-ray-emitting universe, discovering several star-like sources of X-rays and a curious diffuse glow which seemed to come from all directions in the sky.

The X-ray background, as it has become known, is one of the longest-standing mysteries in astronomy. Hundreds of ideas have been proposed for its origin. Among the most plausible is that it comes from a huge population of hidden quasars, super-bright galaxies so shrouded in choking interstellar dust that they are totally invisible to conventional optical telescopes.

Since X-rays are the only type of light that can penetrate such dust, they escape the quasars and account for the cosmic X-ray background.

Now Andy Fabian of the university of Cambridge has presented powerful evidence to support the idea that there is a major population of hidden quasars - as many as 10 hidden quasars for every known one, and the giant black holes which power these bizarre objects could be contributing half as much light as all the known stars put together. "Incredibly, astronomers may have overlooked one of the major sources of the light in the universe," says Fabian.

Quasars are thought to be powered by monstrous black holes, up to 10 billion times as massive as the Sun, which suck in gas and ripped-apart stars. In the process, this material is heated to many millions of degrees, making quasars ferociously bright - hundreds or even thousands of times as bright as a normal galaxy.

There are no quasars around today, though they were common in the past. However, there is strong evidence of very massive dark objects lurking in the hearts of many, if not all, nearby galaxies, including our own Milky Way.

These are generally thought to be dead quasars, giant black holes which in the past burnt brilliantly bright but which have long since run out of fuel and gone out. The dead quasars in the neighbourhood are the key to Fabian's argument because they enable him to estimate the total amount of light they produced when they were active and then compare it with the amount of light being emitted by distant, and therefore ancient, quasars.

Super-hot gas falling on to the black hole at the heart of a quasar can convert between 10% and 40% of its mass into light energy, depending on how fast the black hole is spinning (by comparison, nuclear reactions convert less than 1% of the mass of nuclear fuels into other forms of energy). Fabian simply adds up the masses of the dead quasars in the neighbourhood, calculates the energy equivalent of this mass from Einstein's E=mc2 formula, and multiplies by the efficiency factor of 10% to 40%.

"When you do the sums, the total energy radiated by the quasars in their lifetimes turns out to between 10 and 30 times bigger than can be accounted for by all the known quasars," says Fabian. "Conclusion - there's an awful lot of missing energy."

Deducing that dust-obscured quasars can account for the missing energy is the clever bit. Fabian and his Cambridge colleague Kazushi Iwasawa exploit the fact that X-rays lance out of an obscured quasar without hindrance.

The strength of the X- rays, as deduced from the X-ray background, is therefore the only reliable measure of what the population of obscured quasars is truly emitting.

Fabian uses this X-ray strength to "calibrate" the spectrum of a typical unobscured quasar - that is, determine exactly how much light the population of hidden quasars is emitting at all other wavelengths.

"When you add everything up, it turns out that the hidden quasars account for all the light energy inferred from the dead quasars," says Fabian. "Everything fits."

Fabian points out that the idea that choking dust is a characteristic feature of quasars is supported by evidence from nearby galaxies. "The three nearest active galaxies - scaled-down versions of quasars - all contain large amounts of obscuring dust," he says.

It only remains to confirm Fabian's idea. This involves doing the energy sums for individual obscured quasars and also requires a new generation of X-ray telescopes which can see enough detail in the sky to "resolve" individual quasars.

Fabian believes Nasa's Chandra X-ray observatory will be able to do this in the next few years.

But most of the energy absorbed by the dust is re-radiated as far-infrared light, with wavelengths in the region of a fraction of a millimetre to a millimetre. Resolving individual quasars at such wavelengths may be possible with Britain's Submillimetre Common User Bolometer Array (SCUBA).

However, the task may require new infrared satellites such as Nasa's Space Infrared Telescope Facility, due to be launched in 2001, and ESA's Far-Infrared Space Telescope (FIRST), due for launch in 2007.

If Fabian is right then giant black holes released a tremendous amount of energy in the early universe.

"The powerful winds of particles they produced would have blown the gas from galaxies, terminating their growth," he says. "We may be looking at the missing jigsaw piece which will explain the galaxy shapes we see around us."